DESCRIPTION (Applicant's Description): Our objective is to study a signal transduction process in Drosophila oogenesis that guides a re-organization of the oocyte's microtubule cytoskeleton. The resulting polarized microtubules template the subcellular distribution of asymmetrically localized mRNAs, which define the anterior-posterior axis of the oocyte, later manifest in the embryo. These studies will complement existing molecular insights that have made Drosophila the premiere model organism for understanding how polarity and pattern are generated in a multicellular eukaryote. They will also bring molecular genetic tools to bear on a fundamental aspect of cell biology that is not well understood, namely regulation of the cytoskeleton by external signals. We anticipate that the signaling mechanisms we uncover will be conserved and thus of widespread relevance to human disease. The most immediate impact on human health is likely to come from functional insights into Drosophila genes that are homologous to human genes that are mutated in heritable diseases. Our experimental approach exploits the many classical and molecular genetic technologies that have been devised to study Drosophila development, and takes full advantage of the recent accomplishments of the Drosophila Genome Project. Our major initial aim is to identify key molecules that participate in the regulation of microtubule re-organization in oogenesis. We previously demonstrated that protein kinase A (PKA) is required for this process. We will now look for mutations in other genes that enhance mild polarity defects due to partial loss of PKA activity. We will test if the genes identified by these mutations are essential for correct microtubule re-organization and define these genes molecularly using information from the Drosophila Genome Project. Some of these newly-identified mutations will then be used to perform further, more exhaustive modifier screens to identify a more complete set of relevant genes. The nature of these gene products, their subcellular localization and potential binding partners will be determined and used to formulate and test models for signal-dependent microtubule re-organization. Thus, we aim eventually to determine the nature and source of the signal presented to the oocyte, the signal transduction pathways that are activated, and how these affect the activity or localization of microtubule organizing centers and the growth or shrinkage of microtubules nucleated from those sites.